Asymmetric audiogenic seizures in mice: a possible analogue of focal epilepsy

Localizing and lateralizing values of postictal behavioral impairments in epileptic rats

Transient postictal behavioral impairments in patients with epilepsy provide clues to seizure localization, but no attempt has been made previously to study the localization/lateralization value of postseizure motor disturbances in experimental models of epilepsy. The present study investigated relation of postictal motor deficit to seizure localization in the rat model of sound-induced reflex epilepsy. Sound-induced motor seizures started with a focal brainstem seizure (running) and progressed to a secondarily generalized seizure. Depending on innate or acquired seizure susceptibility of rats, focal brainstem seizures secondarily generalized within the brainstem (brainstem-generalized seizures) or spread to the forebrain (focal or generalized forebrain seizures). All sound-induced seizures were followed by catalepsy and abnormal limb posturing. The duration of the postictal catalepsy and the pattern of the posture abnormality depended on brainstem or forebrain localization of secondarily generalized seizures. Brainstem-driven seizures induced long-lasting whole-body catalepsy and cataleptic limb posture in the postictal period. Secondary seizure generalization to the forebrain led to shortening postictal catalepsy and development of rigid limb posturing. Asymmetric limb posturing was always observed after focal forebrain seizures, and the postictal asymmetry was closely linked to ictal asymmetry of the earliest running seizure phase, predicting lateralization of the seizure-onset side. This is the first demonstration of circuit-specific postictal behavioral impairments and their localization and lateralization values in epileptic rats.

Audiogenic kindling and secondary subcortico-cortical epileptogenesis: Behavioral correlates and electrographic features

Human epilepsy is usually considered to result from cortical pathology, but animal studies show that the cortex may be secondarily involved in epileptogenesis, and cortical seizures may be triggered by extracortical mechanisms. In the audiogenic kindling model, recurrent subcortical (brainstem-driven) seizures induce secondary epileptic activation of the cortex. The present review focuses on behavioral and electrographic features of the subcortico-cortical epileptogenesis: (1) behavioral expressions of traditional and mild paradigms of audiogenic kindling produced by full-blown (generalized) and minimal (focal) audiogenic seizures, respectively; (2) electrographic manifestations of secondary epileptic activation of the cortex - cortical epileptic discharge and cortical spreading depression; and (3) persistent individual asymmetry of minimal audiogenic seizures and secondary cortical events produced by their repetition. The characteristics of audiogenic kindling suggest that this model represents a unique experimental approach to studying cortical epileptogenesis and network aspects of epilepsy. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

Lateral asymmetry of early seizure manifestations in experimental generalized epilepsy

Reorganization of seizure networks during epileptogenesis involves cortico-subcortical and interhemispheric interactions. In the audiogenic kindling (AK) model of generalized tonic-clonic seizures, upstream seizure propagation along ascending brainstem-to-forebrain pathways determines progressive intensification of repeated sound-induced convulsions. Full-blown audiogenic seizures are bilaterally symmetric and their repetition results in bisynchronous recruiting the cortex in secondary epileptogenesis. The present study describes lateral asymmetry of initial behavioral and EEG manifestations of audiogenic seizures and AK in Wistar and WAG/Rij rats with acoustic hypersensitivity. These rats exhibit consistent individual lateralization of running seizures (run directionality) induced by repeated binaural stimulation. Since this initial preconvulsive running reflects seizure onset in the auditory brainstem, the running asymmetry suggests non-symmetric early epileptic activation of brainstem substrates by sound in these rats. Repetition of the asymmetric brainstem seizures led to asynchronous recruiting the cortex into seizure network and lateralization of running seizures was predictive for asymmetry of early cortical seizure manifestations in Wistar and WAG/Rij rats. Both electrographic markers of AK, spreading depression (SD) and post-running afterdischarge, first appeared in the cortex ipsilateral to run direction, suggesting lateralized brainstem-to-forebrain seizure generalization during AK. At the population level, no bias in lateralization of running and SD was found in Wistar and WAG/Rij rats but incidence of secondary cortical seizures varied, depending on strain and run laterality. Among Wistar rats, cortical seizures developed more rarely in right-runners than in left-runners, suggesting enhanced resistance of the right hemisphere to epileptogenesis in rats of this strain. WAG/Rij rats with mixed (absence and audiogenic) epilepsy showed weak lateralization of early cortical seizures and no left-right difference in their incidence during AK. Present findings suggest (1) lateralized brainstem-to-forebrain seizure propagation and hemispheric difference in its facility in Wistar rats, (2) alterations of intra- and interhemispheric seizure propagation in WAG/Rij rats with genetic absence epilepsy.

Audiogenic seizures in unilaterally sensitized and monaurally stimulated Wistar rats

Results of previous studies (Pierson, M. and Swann, J., Epilepsia, 32 (1991) 1-9) have demonstrated that exposure of Wistar rats to noise on day 14 results in audiogenic seizure susceptibility. Experiments reported here examined whether unilateral susceptibility could be induced in rats by monaural restriction of this noise exposure. Behavioral attributes of seizures on day 28 were compared in groups that were: binaurally noise-exposed/binaurally tested, binaurally noise-exposed/monaurally tested, monaurally noise-exposed/binaurally tested and monaurally noise-exposed/monaurally tested. Effects of left- and right-ear exposures and tests were assessed separately. Unilateral susceptibility was evident since seizures could be elicited later only by stimulation of the originally noise-exposed ear. Seizures were behaviorally different in monaurally noise-exposed and binaurally noise-exposed animals. Convulsions, directional reversals during running episodes, and relatively short latencies occur only in binaurally noise-exposed rats. These behaviors occur with either monaural or binaural stimulation. Initial, running direction was random in binaurally stimulated/binaurally noise-exposed rats, but was fixed in all other groups depending on which ear was exposed in either sensitization (day 14) or testing (day 28). Right- and left-ear sensitizations or tests resulted in left-directed and right-directed running onsets respectively. Previous studies of the effect of selective CNS lesions in instances of unilateral or bilateral susceptibility have led to the understanding that seizure initiation in unilaterally susceptible animals is mediated by the crossed ascending auditory pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Monaural and binaural audiogenic seizures in mice

The progression of sound-induced seizures was examined in unilaterally or bilaterally sensitized SJL/J mice tested either monaurally or binaurally. An unexpected right-side advantage for becoming susceptible to audiogenic seizure was observed. In addition, two distinct patterns of seizure progression were noted, a uniphasic sequence in which a single burst of running preceded the convulsion and a biphasic pattern with two such bursts. The biphasic progression is viewed to be the result of unilaterally initiated seizures and characteristically reached only a clonic level of severity. Uniphasic seizures are concluded to be the result of bilaterally initiated seizures and, when they occurred after more than 30 s of auditory stimulation, frequently reached a tonic level of intensity. The present results support the view that audiogenic seizures are characterized by precisely timed, sequential processes dependent upon the specific priming and test procedures employed.

Sound-induced seizure: asymmetries exhibited are related to the latency of seizure onset

Audiogenic seizures of SJL/J mice sensitized before 3 weeks of age were found to be of two distinct types. Seizures with short latencies to onset were without clear asymmetries of either run or fall direction, whereas seizures occurring after a longer latency were characterized by asymmetries of both of these behaviors.

Lateralized audiogenic seizure: motor asymmetries exhibited and the effects of interrupted stimulation

Sensitized SJL/J mice were monaurally tested for susceptibility to sound-induced seizure. The acoustic stimulation was interrupted for 15 s at various points during the biphasic pattern of running which these mice characteristically exhibit before seizing. Asymmetries of run directions and side of fall were noted. In addition, a dramatic reduction in seizure incidence was found when the interruption either extended into the period when the later burst of running would have occurred or began during this running burst. Finally, the results indicate that the latencies to the later running burst and the subsequent seizure are determined at or about the time of the onset of the first burst of running.

Circadian rhythms and temporal structure of the susceptibility to tonic-clonic seizures in the mouse

A circadian paradigm of susceptibility to tonic-clonic convulsions was established for two strains of mice. This research dealt with rhythmometric methods for estimating the main circadian parameters characterizing the temporal structure of these behavioral phenomena. Such experiments provided a quantitative approach for studying some neurobiological mechanisms regulating the seizure susceptibility.

Hippocampal lesions increase the severity of unilaterally induced audiogenic seizures and decrease their latency

The effect of unilateral and bilateral lesions of the hippocampus was examined in unilaterally tested SJL/J mice. The data indicate that lesions of the hippocampus decrease the latency to and increase the severity of audiogenic seizures.